Base station synchronization

ABSTRACT

The present invention is a method for adjusting a timing of at least one first base station to maintain synchronization with a neighboring base station. An estimation of a timing accuracy associated with each said at least one first base station with respect to said neighboring base station is determined. For each of the first base station having its timing accuracy over a threshold, a first message to transmit a communication burst is received by the first base station. The communication burst is then received by the neighboring base station and a measurement of an estimated time difference between the first base station and the neighboring base stations in response to a second message is made. The first base station&#39;s timing is then adjusted in response to the measurement.

[0001] This application is a continuation of application Ser. No.09/826,547, filed on Apr. 5, 2001; which claims priority fromProvisional Application Ser. Nos. 60/223,405, filed on Aug. 4, 2000 and60/195,543, filed on Apr. 7, 2000

BACKGROUND

[0002] The present invention relates generally to digital communicationsystems. More specifically, the invention relates to a system and methodof synchronizing a plurality of base stations in a cellularcommunication network.

[0003] The proposed 3^(rd) generation wireless protocols require anapproach that is ibased on a simple, but costly procedure of requiringeach base station to be externally synchronized to a highly accurateexternal source.

[0004] Techniques which support base station synchronization requirethat a base station passively listen to synchronization transmissionsfrom its neighbors, e.g. the synchronization channel (SCH) or the commoncontrol physical channel (CCPCH), and follow procedures similar to thoseperformed by user equipment (UE) in order to synchronize. Anotherapproach requires each base station to occasionally send a specialsynchronization burst in coordination with one or more of its neighborslistening for the transmission. Yet another approach has UEs measure thetime difference of arrival of transmissions from each of two cells(TDOA). These techniques utilize a precisely accurate source in everybase station. Since each base station has this source, these techniquesare costly and inconvenient.

[0005] Therefore, there exists a need for a system and method thatallows fast, efficient, and less expensive synchronization betweenoperational base stations without consuming additional physicalresources.

SUMMARY

[0006] The present invention is a method for adjusting a timing of atleast one first base station to maintain synchronization with aneighboring base station. An estimation of a timing accuracy associatedwith each said at least one first base station with respect to saidneighboring base station is determined. For each of the first basestation having its timing accuracy over a threshold, a first message totransmit a communication burst is received by the first base station.The communication burst is then received by the neighboring base stationand a measurement of an estimated time difference between the first basestation and the neighboring base stations in response to a secondmessage is made. The first base station's timing is then adjusted inresponse to the measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a block diagram of a communication system.

[0008]FIG. 2 is a block diagram of a radio network controller (RNC) madein accordance with a preferred embodiment of the present invention.

[0009]FIG. 3 is a block diagram of a base station and UE made inaccordance with 6 a preferred embodiment of the present invention.

[0010]FIG. 4 is an illustration of the hierarchal time quality designmade in accordance with a preferred embodiment of the present invention.

[0011]FIGS. 5a and 5 b is a flow diagram of the system in accordancewith a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The preferred embodiments of the present invention will bedescribed with reference to the drawing figures where like numeralsrepresent like elements throughout.

[0013]FIG. 1 illustrates a simplified wireless spread spectrum codedivision multiple access (CDMA) or time division duplex (TDD)communication system 18. The system 18 comprises a plurality of Node Bs26, 32, 34, a plurality of RNCs, 36, 38, . . . 40, a plurality of userequipments (UTE) 20, 22, 24, and a core network 46. A node B 26 withinthe system 18 communicates with associated user equipment 20-24 (UE).The node B 26 has a single site controller (SC) 30 associated witheither a single base station 30, or multiple base stations 30 ₁ . . . 30_(n). Each base station has an associated geographic region known as acell. It should be known that even though base station synchronizationis disclosed, cell synchronization may also be accomplished using thepresent invention.

[0014] A Group of node Bs 26, 32, 34 is connected to a radio networkcontroller (RNC) 36. The RNCs 36 . . . 40 are also connected to the corenetwork 46. For brevity, the following refers to only one node B, butthe present invention can be readily applied to multiple node Bs.

[0015] In accordance with a preferred embodiment, the RNC 36 maintainsbase station synchronization within and between the node Bs 26, 32,34.Referring to FIG. 2, the RNC 36 may request measurements from a basestation 30 ₁ . . . 30 _(n) or UE 20, 22, 24 through its messagegenerator 53; receive measurements through its measure receive device54; optimally update its estimates of states based on these measurementsusing its synchronization controller 55; and manage a set of statesstored in a covariance matrix 57. The stored states are used forsynchronization and represent the time error of each base station 30relative to a reference, the rate of change of each time error, and thetransmission delay between base stations 30.

[0016] The RNC 36 also manages a set of measurements stored in adatabase 59 comprising: time of arrival of a measured waveform (i.e.sync burst); time difference of arrival of transmissions from two basestations as measured by a UE20; and estimates of state uncertainties andmeasurement uncertainties. The RNC 36 uses advanced filtering, such asKalman filters, to estimate parameters that define relative clock drift,and to refine parameters such as exact range between one element andanother. The estimated time drift is used to infer the frequencymismatch between the frequency references of the respective basestations and reasonableness checks to ensure that occasional, grosslyinaccurate measurements do not corrupt the process.

[0017] The RNC 36 assigns a time quality to each base station 30, . . .30. This time quality is measured by the RNC 36 by selecting one basestation as the time base reference for all others. All other basestations are assigned a variable time quality that is updated based onmeasurements and applied corrections. The time quality may be an integer(e.g., 0 to 10). A lower quality value implies a better accuracy.Alternately, the quality may be a continuous (floating point) variable.The reference base station (master base station) is preferably,permanently assigned a quality of 0. All other remaining base stationsare assigned values which vary and are adjusted with respect to thereference base station. To illustrate this time quality hierarchicaldesign, FIG. 4 displays a master base station wherein all base stationsslave 1, slave 2, slave 3, are assigned time quality values which varywith respect to the master base station. In one embodiment the timequality of slave 2 base stations are assigned values which vary withrespect to the slave 1 base stations and slave 3 base stations areassigned values which vary with respect to slave 2 base stations.

[0018] The normal mode of operation of the RNC 36 updates a covariancematrix 57 for the states stored in the RNC database 59, once per apredetermined time unit (e.g. once per five seconds or a time determinedby an operator). One element ofthe covariance matrix 57 is the estimatedvariance of each base station's time error.

[0019] When a base station's time error variance exceeds a predeterminedthreshold, the RNC 36 initiates a message to support that base station'stime error update. The update is performed in one of three ways: first,the subject base station is instructed to measure the base station timeof arrival (BSTOA) of a sync burst from a neighboring base station 30,30 ₂ . . . 30 _(n); second, a neighbor base station 30 ₁, 30 ₂ . . . 30_(n) with better quality is instructed to measure BSTOA of the subjectbase station's transmission; or third, a UE 20 measures the BSTOA ofsync bursts of that base stations and a neighboring base station 30 ₁,30 ₂ . . . 30 _(n).

[0020] In the first and second approaches using base station to basestation BSTOA, the time of arrival of one base station transmission toanother is observed. Referring to FIG. 3, a transmitting base station 30₁ sends a known transmission pattern at a predefined time. Thistransmission pattern may be a sync burst from the sync burst generator62 of the base station 30 ₁, which passes through an isolator 64 priorto being radiated by an antenna 70. The receiving base station 30,detects the transmitted waveform using its measurement device 60 whichoutputs a large value when the received signal coincides with theexpected signature. If the receiver and transmitter were at the samelocation and had precisely synchronized clocks, the output of themeasurement device 60 would occur at the same time as the transmittedwaveform. However, clock misalignment and transmission path delay causesa time difference.

[0021] Transmission path delay is defined as per Equation 1:

R/c+x   Equation 1

[0022] R/c is the distance, R, between a transmitting unit and receivingunit divided by the speed of light, c. The term x accounts for equipmentdelays. When base stations are very far apart the quantity, R/ctypically dominates. Radio waves travel at the speed of light,approximately 1 foot per nanosecond, or 3×10⁸ meters per second. Theobjective of base station synchronization is to align the base stationsto within 1-3 microseconds. Therefore, when base stations are separatedby distances on the order of ½ mile (1 km) or more, the distances aresignificant. However, for pico or micro cells, separated by tens ofmeters, the distances are insignificant compared to the measurementaccuracies, x, which dominates.

[0023] Based on these considerations, when attempting to synchronizebase stations far apart (more than 1 km) the knowledge of the separationis important. When attempting to synchronize base stations within 50meters or so, the exact positions become irrelevant. After themeasurement of BSTOA is performed, the known propagation distance storedin the RNC database 59 is subtracted and the difference is consideredthe misalignment in time between the base stations.

[0024] The third approach measures the relative time difference ofarrival (TDOA) between two transmissions sent by two different basestations as observed by a UE. The UE measures and reports the observedTDOA between transmissions from two base stations. The RNC 36 sends amessage to the UE 20, 22, 24 to measure the TDOA of two base stations.Upon receipt of this message, the UE 20, 22, 24 receives thetransmission of the two base stations, via its antenna 72 and isolator64, and measures the TDOA using the UE measure receive device 68 andtransmits the measurements to its associated base station.

[0025] If the UE position is known (i.e. its range to each of the twobase stations rl and r2 is known) and both base stations timing iscorrect, the time difference of arrival (TDOA) is defined as perEquation 2.

(r1−r2)/c   Equation 2

[0026] Measured deviations from this value would be an indicator of timebase misalignment. As those skilled in the art know, if the ranges r1and r2 are sufficiently small as would be true for pico-sized cells, itwould not be necessary to know their values. Observed time difference ofarrival could be used directly as a measure of time difference oftransmission.

[0027] Once an approach is chosen, the appropriate message istransmitted to either a base station 30 ₁, . .. 3O_(n) or a UE 22, 24,20. If the message is sent to a base station 30 ₂, the base station 30 ₂is told which neighbor to monitor and measure. If the message is to a UE22, the UE 22 is told which base station to measure in addition to itsown base station.

[0028] Referring back to FIG. 2, the RNC 36 has stored the range betweeneach base station 30 ₁ . . . 30 _(n) within its database 59. Itsubsequently checks to see if there is a neighbor base station 30 whichhas abetter time quality than the base station 30 ₂ to be updated. Oncesuch a neighbor base station 30, is found, a message is initiated to theneighboring base station 30, to take a measurement from the “out ofsync” base station 30 ₂. Alternatively, the RNC 36 is able to send amessage to the “out of sync” base station 30 ₂ and request that it takea measurement of the neighboring base station 30 ₁. The requested basestation, for purposes of this embodiment, the “out of sync” base station30 ₂, then takes the measurement ofthe “in-sync” base station 30 ₁ andsends the measured value back to the RNC measurement device 54. The RNCmeasurement device 54 forwards the measured value to the synchronizationcontroller 55 which computes the time of transmission of the measurementby subtracting the propagation time R/C.

[0029] Once the time of transmission is calculated by the RNCsynchronization controller 55, the value is compared to the value storedin the RNC database 59. The RNC synchronization controller 55 thencomputes Kalman filter gains and updates the states in the covariancematrix 57 using the difference between the calculated and predeterminedtime of arrival and the common gains. If the difference is beyond acertain threshold, the RNC message generator 53 will then send anothermessage to the “out of sync” base station 30 ₂ to adjust its time baseor its reference frequency in order to get “in sync” with the other basestation 30 ₃. . . 30 _(n) under the control of the RNC 36.

[0030] The base station 30 ₂ conducts the requested adjustment andreports it back to the RNC measurement device 54. The databases withinthe RNC 36 is updated, including a correction to the subject basestation's 30 ₂ time reference, its time rate of change, an update of itscovariance matrix 57 (including, most significantly, its estimated RMStime error and drift error), and an update to its time quality.Referring to FIG. 4, a base station whose time base is corrected basedon a comparison to another base station, must never be assigned aquality equal to or better than that of a base station to which it is aslave to. This procedure guarantees stability. To illustrate, if a slave2 base station is to be corrected, the slave 2 base station can only beassigned a value less than that of time quality of its slave 1 basestation. This ensures that the time quality of a base station will notsynchronize to a slave base station of the same level or less whichcould eventually lead to a cluster of base stations drifting “out ofsync” with the master base station.

[0031] As disclosed earlier, another approach of taking measurements inorder to adjust the “out of sync” base station 30 ₂ uses an UE 20, 22,24. If this method is chosen by the RNC 36, a message is sent to the UE22 to measure the sync burst of the “out of sync” base station 30 ₂ andthe “in sync” base station 30 ₁. Once the measurement is taken by the UE22, the measurements are sent to the RNC 36 and processed. Similar tothe methods described above, the measurements are compared to the knownmeasurements stored in the RNC database 56 and covariance matrix 57 andan adjustment measurement sent to the “out of sync” base station 30 ₂.

[0032] The flow diagram of the system in accordance with the preferredembodiment is illustrated in FIGS. 5a and 5 b. The RNC 36 updates thecovariant matrix 57 and database 59 once per unit time (step 501). Whenthe RNC 36 detects that a base station's 30 ₂ . . . 30 _(n) time errorvariance exceeds a predetermined threshold (step 502), the RNC 36decides whether to use a base station to measure BSTOA or a UE tomeasure TDOA in order to update the “out of sync” base station's timeerror variance (step 503). If the RNC 36 decides to measure BSTOA, amessage is sent to a neighboring base station ofthe “out of sync” basestation to measure the base station time of arrival, or the message issent to the “out of sync” base station to measure the time of arrival ofthe neighboring base station (step 504). The appropriate base stationtakes the necessary measurement (step 505) and transmits the measurementto the RNC 36 (step 506). If the RNC 36 decides to measure TDOA, the RNCC36 sends a message to a UE to measure the time difference of arrival oftwo base stations (step 507 a), one being the “out of sync” basestation. The UE measures the TDOA of each base station (step 507 b) andsends the difference of these measurements to the RNC 36 (step 507 c).Upon receipt by the RNC 36 of the appropriate measurements (step 508),the RNC 36 compares the measurement to the value stored in the RNCdatabase 59 (step 509). If the difference is beyond a certain threshold,the RNC 36 sends a message to the “out of sync” base station to adjustits time base or its reference frequency (step 510) in accordance withthis difference. The “out of sync” base station conducts the requestedadjustment (step 511) and reports it back to the RNC 36 (step 512). TheRNC database 59 and covariance matrix 57 are then updated to incorporatethe new values (step 513).

[0033] A preferred embodiment is a system and method that resides ineach RNC 36. In the prior art, a controlling radio network controller(C-RNC) communicates directly with its base stations and a serving radionetwork controller (S-RNC) communicates directly with its ULs. For caseswhere neighboring base stations are under control of different radionetwork controllers (RNC), there may be a need to add communicationbetween the C-RNCs and S-RNCs that control the neighboring base stationsand UEs.

[0034] An alternative embodiment requires each pair of base stationsthat can hear each other to move its own frequency closer to that of theother. The relative amount of adjustment is defined by a set of uniqueweights which are assigned to each base station and stored in the RNCdatabase 59. The process of adjusting each of the base stations is thesame as disclosed in the preferred embodiment above except that both the“in sync” and “out of sync” base stations are adjusted based on theweights assigned to the respective base stations. With differentweights, one can achieve varying degrees of centrality, between thefully central to the fully distributed.

[0035] The most preferred embodiment enables an RNC 36 to send timecorrections and/or frequency corrections to a base station 30,...30,.The master base station is responsible to ensure that each of its basestations have a time reference slaved to it, accurate within a specifiedlimit. The RNC 36, in its algorithms and corrections, assumes that thereis negligible error existing between the master base station and itsbase stations and therefore assumes that all base stations have the sametime reference.

[0036] As a consequence, the RNC 36 does not attempt to estimate theindividual time errors between the master base station and its basestations and the master base station must eliminate or compensate fortiming errors between the master base station and each of the other basestations, since the associated RNC 36 does not perform a correction.This embodiment presents a clean interface between an RNC 36 and amaster base station. It enables the master base station to apply its ownsolution to slave synchronization which is well suited to pico-cells.

[0037] In an alternative embodiment, each base station has anindependent time and frequency reference which enables an RNC 36 to sendtime corrections and/or frequency corrections to each base station. TheRNC 36, in its algorithms and corrections, estimates the states whichrepresent the time and frequency error of each base station.

[0038] As a consequence, the RNC 36 attempts to estimate the individualtime errors between each base station and the master base station,measurements involving one base station provide no benefit to estimatingthe states of another base station. Therefore, the base stationmanufacturer need only provide loosely bounded errors in the timing andtime drift of the base stations, and every base station must have anacceptable connectivity over the air to another base station (same ordifferent base station).

[0039] This alternative embodiment benefits large cellular areas wherethe distance between base stations are far. The ability to correct onebase station slaved to the time reference of a master base stationthrough measurements involving another base station slaved to the samemaster base station is limited.

[0040] Each base station in this alternative embodiment uses independenttime references but the master base station provides a frequencyreference. An RNC 36 sends time corrections for each base stationindividually and/or a single frequency correction to a master basestation. The RNC 36 ensures that the clock of each base station isslaved in frequency to the clock of the master base station. The RNC 36,in its algorithms and corrections, assumes that there is negligibledrift error between the master base station and its assigned basestations, but estimates offsets which are treated as constant.

[0041] As a consequence, the RNC 36 estimates the individual time errorsbetween the master base station and its base stations and the commonfrequency drift of the base stations with regard to the master basestation.

[0042] This alternative embodiment has features similar to thosedescribed in the previous alternative embodiment where base stationsthat are far from the master base station benefit. This embodimentprovides a mechanism to remove time mismatches in long distances. Takingadvantage of the assumption that these time offsets are stable, thisembodiment takes advantage of a measurement involving any base stationslaved frequency to the clock of the master base station, to update thedrift rate for all base stations slaved to the same master base station.

[0043] Another alternative embodiment has the RNC 36 providing estimatesto the master base station to support its synchronization of the basestations slaved to it. An RNC 36 sends time corrections and/or frequencycorrections for each associated base station to its respective masterbase station. The master base station ensures that its associated basestations each have a time reference slaved to itself, accurate within aspecified limit. The master base station may elect to use the basestation-unique estimates to aid in the base station synchronization. TheRNC 36, in its algorithms and corrections, creates a best estimate ofthe time and frequency error between the master base station and itsbase stations. In performing state estimates it weighs the relativeconfidence between the measurements and the base station erroruncertainty.

[0044] As a consequence, the RNC 36 attempts to estimate the individualtime errors N between the master base station and its base stations, andthe master base station eliminates and/or compensates for timing errorsbetween the master base station and each base station slaved to its timereference, or requests assistance from the RNC 36.

[0045] While the present invention has been described in terms of thepreferred embodiments, other variations which are within the scope ofthe invention as outlined in the claims below will be apparent to thoseskilled in the art.

What is claimed is:
 1. A method for adjusting a timing of at least onefirst base station to maintain synchronization with a neighboring basestation, comprising the steps of: determining an estimation of a timingaccuracy associated with each said at least one first base station withrespect to said neighboring base station; for each said first basestation having its timing accuracy over a threshold: receiving a firstmessage at that first base station to transmit a communication burst;receiving said communication burst at said neighboring base station;measuring an estimated time difference between that first base stationand said neighboring base station in response to a second message usingsaid received communication burst; and adjusting that first basestation's timing in response to said measurement.
 2. The method of claim1 further comprising the steps of: determining a base station having atime base more accurate than each said first base station, the moreaccurate base station making said measurement; and updating each saidfirst base station's estimated timing accuracy, wherein the updatedtiming accuracy estimation indicates a worse timing accuracy than saidneighboring base station.
 3. The method of claim 2 wherein saidneighboring base station measures the time of arrival of saidcommunication burst to determine said estimated time difference.
 4. Themethod of claim 2 wherein a main base station is assigned a best timingaccuracy and all other base stations within a group comprising each saidfirst base station slave their timing to the main base station.
 5. Themethod of claim 1 wherein said first and second messages are transmittedby a radio network controller (RNC).
 6. The method of claim 5 whereinsaid adjustment is made in response to a third message from said RNC. 7.The method of claim 5 wherein said RNC stores and updates saiddetermined estimation.
 8. The method of claim 7 wherein said RNCcombines any of said first, second or third messages with another ofsaid messages.
 9. A method for adjusting a timing of at least one firstbase station to maintain synchronization with a neighboring basestation, comprising the steps of: determining an estimation of a timingaccuracy associated with each said at least one first base station withrespect to said neighboring base station; for each said first basestation having its timing accuracy over a threshold: receiving a firstmessage at said neighboring base station to transmit a communicationburst; receiving at that first base station said communication burst;measuring an estimated time difference between that first base stationand said neighboring base stations in response to a second message usingsaid received communication burst; and adjusting that first basestation's timing in response to said measurement.
 10. The method ofclaim 9 wherein that first base station measures said time difference.11. The method of claim 10 further comprising the steps of: determininga base station having a time base more accurate than each said firstbase station; and updating each said first base station's estimatedtiming accuracy, wherein the updated timing accuracy estimationindicates a worse timing accuracy than said neighboring base station.12. The method of claim 11 wherein said first base station measures thetime of arrival of said communication burst to determine said estimatedtime difference.
 13. The method of claim 12 wherein a main base stationis assigned a best timing accuracy and all other base stations within agroup comprising each said first base station slave their timing to themain base station.
 14. The method of claim 11 wherein said first andsecond messages are transmitted by a radio network controller (RNC). 15.The method of claim 14 wherein said adjustment is made in response to athird message from said RNC.
 16. The method of claim 14 wherein said RNCstores and updates said determined estimation.
 17. The method of claim16 wherein said RNC combines any of said first, second or third messageswith another of said messages.